Chemical analyzer optical probe and method of manufacturing same

ABSTRACT

An instrument that analyzes chemical properties of a specimen includes a probe that is used in place on the specimen for spectroscopic analysis. The probe has a probe body with a proximal end and a distal end, and a first optical fiber extends from the proximal to the distal ends. A temperature sensor is included in the probe body and is used so insure that the probe does not exceed a rated temperature limit or to monitor specimen temperature while, simultaneously, chemical composition information of the specimen is transmitted by the optical fiber. The probe can be inserted into a container holding the specimen and can yield both temperature and chemical composition information. The probe includes a plurality of metal coated fibers with the distal ends of the fibers positioned inside a bore in the probe. Braze material is placed on the probe near the bore and the braze material is then brazed to the metal coating on the ends of the fiber.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from Provisional Application Ser. No.60/040,769, entitled CHEMICAL ANALYZER OPTICAL PROBE AND METHOD OFMANUFACTURING SAME, filed Mar. 14, 1997 by O'Conner et al. Thisapplication is related to the following copending applications which areincorporated herein by reference and filed on even date herewith:“Improved Rayleigh Backscatter Control Apparatus and Method” (Ser. No.09/038,348 filed Mar. 11, 1998), “Improved Low Noise Raman AnalyzerSystem” (Ser. No. 09/038,438 filed Mar. 11, 1998), and “ChemicalAnalyzer With Free Space Communication Link” (Ser. No. 09/038,438 filedMar. 11, 1998), all having common ownership and inventorship with thepresent application; and “Raleigh Backscatter Control Apparatus andMethod” (Ser. No. 08/947,816 filed Oct. 9, 1997;), and “Method ForStandardizing Raman Spectrometers To Obtain Stable And TransferableCalibrations” (Ser. No. 08/947,689 filed on Oct. 9, 1999;), assigned toEastman Chemical Co.

BACKGROUND OF THE INVENTION

The present invention relates to instruments that analyze chemicalproperties of a specimen using optical means. More specifically, theinvention relates to probe design and construction for such instruments.The invention has particular application to such instruments that detectRaman-scattered light from the specimen.

BRIEF SUMMARY OF THE INVENTION

One aspect of the invention relates to a novel in situ probe forspectroscopic analysis. The probe has a probe body with a distal end anda proximal end, and a first optical fiber extending from the proximal tothe distal end. Advantageously, a temperature sensor is included in theprobe body and disposed proximate the distal end. The temperature sensorcan be used for example to ensure that the probe does not exceed a ratedtemperature limit, or to monitor a specimen temperature whilesimultaneously analyzing a chemical composition of the specimen via theoptical fiber. Hence, one insertion point into a container holding thespecimen can yield both a temperature and chemical compositionmeasurement. Preferred embodiments include additional optical fibersdisposed in the probe body, and connectors held at the proximal end ofthe probe. A preferred temperature sensor is a thermocouple.

Another aspect of the invention relates to a method of constructing afiber optic probe. A plurality of metal-coated fibers having proximaland distal ends are provided, and the distal ends are positioned insidea bore in the probe end. The probe end has a metal coating. Brazematerial is placed on the probe end near the bore, and the assembly isheated in a vacuum oven. In a preferred embodiment an anti-wicking agentis applied to the fibers in a zone near the distal ends prior toheating. Also in the preferred embodiment the metal coating on the probeend is limited to a vicinity in, or in and around, the bore. The brazematerial is preferably a binary alloy of silver and copper.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a system diagram of a preferred chemical analyzer inaccordance with an aspect of the invention;

FIG. 2 depicts chief components of a preferred optical filter used inthe analyzer of FIG. 1;

FIG. 3 is an enlarged end view of optical fibers in the probe shown inFIGS. 1 and 5;

FIG. 4 is a perspective end view of a male fiber connector showing anarrangement of optical fibers at the connector end as used in theanalyzer of FIG. 1;

FIG. 5 is a partially sectional view of a preferred optical probeuseable with the analyzer of FIG. 1;

FIG. 6 is a sectional view of a distal end of the probe of FIG. 5 duringprobe fabrication;

FIG. 7 is a block schematic of a preferred signal conditioning circuituseable with the analyzer of FIG. 1;

FIG. 8 is a schematic of a comparator/asymmetric discriminator circuitdepicted as a block in FIG. 7; and

FIG. 9 is a timeline showing signals at different points in the signalconditioning circuit of FIG. 7.

For convenience, items in the figures having the same reference symbolare the same or serve the same or a similar function.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein, “optic” (al) and “light” refer to electromagneticradiation, whether or not visible to the human eye.

FIG. 1 shows a preferred analyzer 10 that evaluates a specimen ofinterest 12 and provides on a computer 14 or other suitable outputmedium an analyzer output indicative of the presence or amount of one ormore chemical constituents of the specimen. The analyzer 10 illuminatesthe specimen 12 with narrow-band light, collects backscattered lightfrom the specimen, optically isolates a Raman scattering component fromthe backscattered light, and evaluates the Raman scattering component tocalculate the analyzer output. Simultaneously, the analyzer isolates aRayleigh scattering component from the backscattered light. If theRayleigh scattering component falls below a threshold level, which mayresult for example from disconnecting a fiber connector or withdrawingthe probe from the specimen, the narrow-band light illumination is shutoff. This shut off technique is most effective in specimens such asliquids that have significantly higher Rayleigh scattering levels thanthat of gasses such as air.

A diode laser 16 launches essentially monochromatic, narrow-band lightinto a fiber 18 a of a 2-by-2 beamsplitter 20. A wavelength of about 810nanometers (nm) for the narrow-band light has been found satisfactory.Shorter wavelengths increase the amount of Raman scattering, but mayalso produce unwanted fluorescence in some specimens; longer wavelengthsare less likely to produce fluorescence but yield a lower Raman signal.In practice, a diode laser having a wavelength between about 750 and 850nm is preferred. The selected wavelength should not however coincidewith an absorption line of the specimen 12, if maximum Raman scatteringlevels are desired. The laser can have a multimode output and be capableof emitting 700 mW to 1.2 W of optical power during analyzer operation.Laser 16 also includes a driver circuit with a control input at line 24,and a temperature control circuit if the laser source is a diode laser.The control input of laser 16 controls the amount or intensity of narrowband light injected into fiber 18 a.

Beamsplitter 20 divides the laser light launched into fiber 18 a betweenfibers 18 b, 22 a, preferably in equal amounts although other ratios arealso contemplated. The narrow band light passes from fiber 18 b to afiber 18 c via a connector pair 26. Connector pair 26 includes a maleconnector end holding each fiber end, the connector ends facing eachother inside an alignment bushing. SMA-type connector pairs arepreferred for robustness and ease of modification to nonstandard boresizes, but other known styles such as ST or FC are also contemplated.Fiber 18 c connects to a bandpass filter 28 in a fibertermination/filter module 30. Laser light passes through filter 28 to afiber 18 d, which carries the light to a probe 32 adapted to contact thespecimen 12. Fibers 18 c,18 d are preferably part of armored cableassemblies 34,36 respectively.

The analyzer 10 is preferably arranged as a main analyzer unit 38situated in a control room (or other suitable location that can providethe necessary electrical power), a probe 32 located at the specimen, andthe termination/filter module 30 located near the probe. Main analyzerunit 38 is preferably housed in an intrinsically safe enclosure,configured with an industry standard Z-purge capability at a port 38 athat maintains a positive air pressure inside unit 38 relative to itssurroundings. Armored fiber cable assemblies 34,36 connect unit 38 tomodule 30, and module 30 to probe 32, respectively. Cable assembly 34can be tens or hundreds of meters long.

Optical fiber can itself generate Raman scattering and/or fluorescence(hereinafter, “spurious light signals”) from laser light passing throughit, which if detected can be confused with Raman scattering from thespecimen. The spurious light signals are a function of the fiberproperties (most importantly its length, but also including corematerial, cladding material, and buffer layer material), and generallyhave wavelengths longer than the laser wavelength. Therefore, bandpassfilter 28 is provided in the termination/filter module 30, and module 30is located as close to the probe as possible to minimize fiber 18 dlength and thus minimize any spurious light signals generated in fiber18 d. In a benign environment, the cable assembly 36 (including fiber 18d) can be eliminated and the filter 28 and the other filter (74,discussed below) can be mounted directly on the proximal end of theprobe 32. However, in many practical applications the temperature orchange in temperature encountered even at the proximal end of the probecan have adverse effects on filter performance. Hence, module 30 mountedaway from probe 30 provides a more stable temperature environment forthe filters 28,74. Bandpass filter 28 blocks the spurious light signalsoriginating in fibers 18 a, 18 b, 18 c, from reaching fiber 18 d, butpasses narrow band light from laser 16.

Spurious light signals can be further reduced by using silica-basedfiber having an inert metal buffer layer such as gold. Such fibers havesignificantly lower spurious light signals than similar fibers having apolymer-based buffer layer such as polyimide The relatively high cost ofthe metal-coated fibers however can make it impractical to use themexclusively in analyzer 10, depending on the distances involved.Instead, the use of filters 28,74 permits lower cost, polymer-basedfibers to be used between the main analyzer unit 38 and the filters, andthe preferred gold-coated fibers to be used in probe 32 and betweenprobe 32 and the filters.

A preferred embodiment of filter 28 is depicted in FIG. 2. Male fiberconnectors 39 a,39 b hold the ends of fibers 18 c,18 d against0.25-pitch gradient index (GRIN) lenses 40 a,40 b respectively. A filter42 sandwiched between lenses 40 a,40 b provides the desired spectralfiltering characteristics. Filter 42 preferably comprises aninterference-type filter.

Turning again to FIG. 1, fiber 18 d releasably connects to probe 32 by aconnector pair 44 located at a proximal end 32 a of probe 32 and passesnarrow band light to a fiber 18 e that extends from connector pair 44 toa distal end 32 b of probe 32. Also housed in probe 32 are a fiber 46 aand a group of five fibers 48 a. The line representing fibers 48 a, andsome other lines in FIG. 1, are shown thickened to indicate multipleoptical fibers in the preferred embodiment. Fiber 46 a connects to aconnector pair 50 at proximal end 32 a, and at distal end 32 b is brazedor otherwise held in position together with fibers 18 e and 48 a. At end32 b, the fibers are polished to a mirror finish and arranged as shownin FIG. 3. A braze material 52 holds the fiber ends in place andhermetically seals the probe at end 32 b. The probe design andconstruction is discussed further below.

Narrow band light exits fiber 18 e and illuminates specimen 12 in adetection zone 54 defined by the numerical aperture of fiber 18 e. Thesurrounding six fibers (46 a and 48 a) collect some of the backscatteredlight, which will include a relatively strong Rayleigh component (samewavelength as the narrow band light) and a relatively weak Ramancomponent (longer wavelength than the narrow band light). As will beseen, the preferred analyzer 10 uses the multiple fibers 48 a to boostthe detected Raman component and the single fiber 46 a to sense theRayleigh component for continuity.

Backscattered light traveling down fiber 46 a is directed to a detector56 via fibers 46 b-e, connector pairs 58,60, and a bandpass filter 62.Filter 62 passes the narrow band light wavelength and is substantiallyidentical to previously described filter 28. The purpose of filter 62 isto prevent sunlight, room light, or any other extraneous light collectedby fiber 46 a from being mistaken for Rayleigh backscattered light.Filter 62 also has the effect of preventing the weaker Raman component,if present, from reaching detector 56. An amplifier 64 couples todetector 56 to provide an amplified detector output on line 66.

Advantageously, the detector 56 output, representative of the Rayleighscattering component, is fed back through a signal conditioning circuit68 to the laser control input at line 24. Circuit 68 compares thedetector output on line 66 with a predefined threshold. If the detectoroutput is above the threshold, indicating that the analyzer opticalsystem is intact, circuit 68 provides an output on line 24 thatmaintains laser 16 at its normal, relatively high, output level. If onthe other hand the detector output is below the threshold, indicating afiber disconnection or break, or removal of the probe from the specimen,circuit 68 provides an output on line 24 that shuts off laser 16 or atleast controls it to a lower intensity level. This lower intensity levelcan be set such that the light intensity emitted from probe fiber 18 e,and preferably from fiber 18 b, is within BSI/EN 60825-1 class 1operation (i.e., nonhazardous direct viewing). In this way, analyzer 10can operate with high laser light levels during normal operation andautomatically shut down if a discontinuity is sensed by detector 56,thereby avoiding hazardous viewing by an operator.

Several enhancements to the basic shut-down technique are also providedby signal conditioning circuit 68. One enhancement is the ability todiscriminate between transient losses in the Rayleigh scatteringcomponent, such as may be caused by small bubbles 69 of air or other gaspassing through the detection zone 54, and longer lived losses in thesignal which may result from fiber disconnection or break, or awithdrawal of probe 32 from the specimen. The circuit 68 continuesdriving laser 16 at its high operational intensity level in the presenceof the truly transient losses, but shuts the laser down to the lowerintensity level for the longer-lived losses. This discriminationfunction prevents unnecessary and annoying shutdowns during operation ofanalyzer 10. Another enhancement involves periodically interrogating thelaser 16 after a shutdown has occurred, or at startup, so that if systemintegrity is restored the analyzer 10 will automatically return tonormal operation (i.e., high laser intensity level). These capabilitiesof circuit 68 are described in more detail below.

Turning again to probe 32 in FIG. 1, fibers 48 a are unsupported inprobe 32 except at distal end 32 b, where they are arranged around theemitting fiber as shown in FIG. 3, and at the proximal end 32 a, wherethey are bundled together at a connector pair 70. One of the maleconnector ends 70 a of pair 70 holds the five fibers 48 a as shown inthe perspective end view of FIG. 4. The other male connector end of pair70 holds a single fiber 48 b in alignment with the fibers 48 a, wherefiber 48 b has a diameter sufficiently large to capture light emittedfrom all of the fibers 48 a. For example, if fibers 48 are 100 μm (corediameter), fiber 48 b can be about 300 μm (core dia.). This arrangementgreatly simplifies analyzer 10 interconnections: rather than fiveseparate fibers, connector pairs, and filters connecting the Ramanpickup fiber channel from the probe 32 to the main analyzer unit 38,only one-fifth of those components are required by using a large fiberto collect light from fibers 48. Backscattered light is carried by fiber48 b to the entrance slit of an optical spectrograph 72 via a long passfilter 74, fibers 48 c-e, and connector pairs 76,78. Long pass filter 74has the same construction as the bandpass filter shown in FIG. 2 exceptthat the filter element 42 is fabricated to block the narrow band lightof laser 16 and pass longer wavelengths. Preferably the spectraltransmission of filter 74 is less than 10⁻⁶ at the laser wavelength 810nm and rises to half of its peak transmittance (70% typ. peak trans.) atabout 833 μm. As discussed, fiber termination/filter module 30 ismounted close to probe 32 to keep fiber 48 b short (generally no morethan a few, and preferably <1 meter), so that no spurious light signalscan be produced by Rayleigh backscattered light in fiber 48 b. Filter 74blocks any Rayleigh backscattered light from reaching fibers 48 c-e.Fiber 48 b preferably has a metal (gold)-buffer layer.

Fibers 18 a-e, 46 a, 48 a, 48 e, and 22 a-d are preferably relativelysmall diameter (e.g. 100 μm core) fibers, while fibers 48 b-d arepreferably relatively large diameter (e.g. 300 μm core) fibers. Fibers46 b-e can be either small or large diameter, but preferably are nosmaller than fiber 46 a. All can be graded-index or, preferably,step-index for increased light levels. Fibers 48 e are held at connectorpair 78 in a substantially circular pattern (similar to FIG. 4) foroptimal coupling to fiber 48 d, while at the entrance slit tospectrograph 72 they are held in a linear array. Fibers 18 d, 18 e, 46a, 48 a, and 48 b all have inert metal buffer layers, preferably gold.

A diamond reference 80 is provided in main analyzer unit 38. Fibers 18a, 22 a,22 b carry narrow band light from laser 16 to the surface ofdiamond 80. Bandpass filter 82, substantially identical to filters 28and 62, blocks fiber-generated Raman scattering. Six fibers 22 csurround fiber 22 b at the diamond surface (similar to FIG. 3) tocapture backscattered light from diamond 80. A longpass filter 84,substantially identical to filter 74, blocks Rayleigh scattered lightfrom fibers 22 d. Fibers 22 d, six in number, are arranged circularly atfilter 84 and linearly at the spectrograph 72 entrance slit.

The linear arrays of fibers 22 d and 48 e are arranged colinearly, oneabutting the other, at the entrance slit to spectrograph 72.Spectrograph 72 is preferably equivalent to model SP-150 available fromActon Research Corp., and has a ruled grating with 400 grooves/mm andblazed at 750 nm. A detector array 74, preferably 750 pixels wide by 240pixels high, intercepts and simultaneously monitors the spatiallyseparated Raman scattered light spectra from the specimen 12 and fromthe diamond reference 80. The output from detector array 74 is fed tocomputer 14 over a line 75. Signal processing software residing incomputer 14 is used to produce a standardized Raman spectrum of thespecimen (see copending application of Eastman Chemical Co. entitled“Method For Standardizing Raman Spectrometers To Obtain Stable AndTransferable Calibrations”, filed Oct. 9, 1999, now U.S. Pat. No.5,550,623, incorporated herein by reference) using the diamond Ramanspectrum. Pattern recognition software also residing in computer 14calculates the chemical composition of specimen 12 from the standardizedRaman spectrum of the specimen and calibration training data. Suchpattern recognition software is available from Galactic Industries,Boston, Mass.

As previously mentioned, main analyzer unit 38 is preferablyintrinsically safe. Although computers having intrinsically safekeyboards and monitors are commercially available, there are significantdifficulties in providing a convenient and aesthetic user interfaceusing these components. Therefore, computer 14 is preferably equippedwith a transceiver 86 such as an antenna or an infraredtransmitter/receiver. A user can send instructions to and receiveinformation from computer 14 using a second device such as a laptopcomputer 88 equipped with a similar transceiver 90. Such communicationpreferably occurs over a wireless, fiberless free space path 91,allowing the user to freely move from place to place with computer 88and permitting greater flexibility and choice in a mounting location formain analyzer unit 38. Preferred transceivers 86,90 are commerciallyavailable radio LAN cards for desktop or laptop computers, for examplethe WAVELAN card available from AT&T Lucent Technologies, designed tointerface to a standard PC/MCIA slot or Industry Standard Architecture(ISA) bus slot. Transceiver 86 is depicted in FIG. 1 as such an antennadevice, that partially extends out of the housing of unit 38 andconnects to computer 14 by a coax line 86 a. The transceiver can also bean infrared emitter/receiver disposed inside the housing of unit 38behind a window. Computer 88 has a keyboard and a mouse that are used tosend queries and commands to computer 14. Computer 88 also has a displayto graph or otherwise show the analyzer output data transmitted fromcomputer 14. With this arrangement, computer 14 is preferably equippedwith neither a display screen, a keyboard, nor a mouse, to satisfyintrinsic safety requirements as well as to reduce the size, weight, andelectrical requirements of main analyzer unit 38.

Probe 32 is described in more detail in connection with FIGS. 5 and 6,together with previously discussed FIGS. 1, 3, and 4. Probe 32preferably has a probe body with three main components: a terminus 32 c,a shank 32 d, and a connector housing 32 e, all made of 316 stainlesssteel or other suitable inert materials capable of withstanding severalhundred degree C temperatures, such as Hastelloy C. Terminus 32 c, shank32 d, and housing 32 e are rotationally symmetric about a probe axis 32f, and are connected by braze joints 92,94 as shown. The outer diameterof terminus 32 c, braze joint 92, and shank 32 d is polished to a smoothfinish (0.25 in. dia.) to permit sealing with a ferrule inside the boreof a standard pipe fitting, such as those available from Swagelok Corp.,or the bore of some other container that holds specimen 12.

Fibers 18 e, 46 a, and 48 a (only one of which is shown in its entiretyin FIGS. 5 and 6 for simplicity) extend from their respective maleconnector ends 44 a, 50 a, and 70 a at proximal probe end 32 a to distalend 32 b. Each of the fibers are step-index, with silica or doped silicacore/cladding, and have a thin outer buffer layer of gold, nickel, orother inert metal along their entire length. Male connector ends 50 a,44 a, and 70 a are fixed to connector housing 32 e to permit probe 32 tobe conveniently disconnected and reconnected to cable assembly 36 forease of installation and servicing. Also affixed to housing 32 e is aconnector 96 for a temperature sensor 98 included in probe 32.Temperature sensor 98 is preferably disposed proximate distal end 32 bfor diagnostic purposes to ensure that probe 32 does not exceed itsrated temperature. Alternately, the output of sensor 98 can be used as arough indication of the specimen temperature, whereupon probe 32 takeson a dual role as a fiber optic chemical analysis probe and a specimenthermometer. Although known fiber optic temperature sensors can be usedfor sensor 98, electrical sensors are preferable for their simplicity,and most preferable is a thermocouple (e.g. type K) for its low-cost andreliability. The output of sensor 98 can be monitored with a portable,hand-held device coupled directly to connector 96, or with computer 14,in which case an additional channel such as a twisted wire pair can beincluded in cable assemblies 36,34.

The procedure for brazing fibers 46 a, 18 e,48 a into the stainlesssteel terminus 32 c will now be described. To enhance adhesion, terminus32 c is plated with gold 100 or other metal matching the metal bufferlayer of the fibers. The gold plating extends inside a bore 104 and inthe vicinity thereof, but preferably is removed from or not provided onthe remaining surfaces of terminus 32 c. This is to keep molten brazematerial in the vicinity of bore 104 during fabrication, preventing itfrom spreading over the entire terminus 32 c. An anti-wicking agent orstop-flow substance, preferably a suspension of magnesium hydroxide inwater, is applied to each of the fibers in a zone indicated generally at106 prior to brazing. Zone 106 approaches but does not touch the distalportion of the fibers that extend into bore 104. The anti-wicking agentinhibits the flow of molten braze material along the fiberssubstantially beyond bore 104. With the fibers and terminus 32 c soprepared, the assembly is positioned in a vacuum oven 108 as shown inFIG. 6, with a small ring or loop of solid braze material 110 resting ontop of terminus 32 c at or near bore 104. Preferred braze materials forplatings 100 made of gold are cadmium-free varieties; widely availablebraze type (AWS) BAg-8, a binary alloy composed of about 72% silver and28% copper, is most preferable. The vacuum oven 108 is then heated to atemperature sufficient to melt braze material 110. By preparing thefibers and terminus 32 as described, the molten braze material does notrun out of but rather tends to stay in and around the vicinity of bore104, wicking between the fibers and filling the spaces between them.Upon cooling, the braze material forms a solid hermetic seal within bore104, uniformly filling the inter-fibral spaces inside bore 104 with fewor no voids (see FIG. 3).

As final fabrication steps, a sleeve 112 of terminus 32 c is brazed toshank 32 d using localized heating, and shank 32 d is then brazed toconnector housing 32 e also using localized heating. The sleeve 112partially isolates the brazed fibers in bore 104 from heat generatedduring brazing of terminus 32 c to shank 32 d. Braze material BAg-8 isused for all braze joints. The fiber ends are polished to a flat,mirror-smooth finish at distal end 32 b. Lastly, the other fiber endsare potted into the male connector ends, which connector ends are alsobrazed to connector housing 32 e at proximal end 32 a.

FIG. 7 depicts in block schematic form a preferred signal conditioningcircuit 68. Circuit 68 receives on line 66 the amplified detectoroutput, representative of the Rayleigh scattering component fromspecimen 12, and provides on line 24 an output that controls a lightoutput level of laser 16. A comparator/asymmetric discriminator circuit114 compares the amplified detector output to an adjustable internalthreshold. The threshold is adjusted according to the desired laseroperational output level, fiber attenuation losses, filter and connectorpair losses, and specimen scattering characteristics, to a level lessthan an output level on line 66 for a fully intact system with the probecontacting the specimen, and greater than a lower output levelcorresponding to the amount of Rayleigh scattered light received whenthe probe is withdrawn from the specimen and pointed into the air, orwhen one of the fiber connector pairs is uncoupled. The output ofcircuit 114 feeds into an OR gate 116 and a latch 118 as shown. OR gate116 in turn drives a FET transistor 120 which connects directly orthrough one or more buffer amplifiers if desired to line 24. Thus, aslong as analyzer 10 is intact and probe 32 is disposed in the specimen,the signal on line 66 will be higher than the threshold level, theoutput of circuit 114 will be “HI”, the output of OR gate 116 will be“HI”, turning transistor 120 “ON” to couple the +12V voltage to line 24,thereby maintaining laser 16 at its high operational output intensity.If the probe 32 is withdrawn from the specimen, however, the output ofcircuit 114 will go “LO”, as will OR gate 116, turning off transistor120 and forcing laser 16 to a lower (preferably zero) intensity level.

Circuit 114 also preferably performs a discrimination function againsttransient losses of the detected Rayleigh scattering component. Thisfunction is described in connection with FIG. 8.

The latch 118 is provided so that computer 14 can monitor the activityof circuit 114. An output line 118 a conveys the status of the latch tothe computer, and a reset line 118 b permits the computer to reset thelatch.

Advantageously, circuit 68 also includes a low duty cycle pulsegenerator 122 that also feeds into OR gate 116. In a preferredembodiment, a pulse having a 5 millisecond (ms) duration is generated ata 1 Hz repetition rate. When the laser 16 is in its zero or lowintensity state, as it would be on power-up of analyzer 10 and as itwould be after a drop in the detected Rayleigh scattering componentbelow the threshold, each pulse from generator 122 causes the laser tomomentarily (for the duration of the pulse) provide the higher outputintensity. The pulses are kept short enough, and the duty cycle smallenough, to keep the light emitted from probe 32 or even from connectorpair 22 below the safety limits for the human eye and for explosiveatmosphere environments. When system integrity returns to analyzer 10,the Rayleigh backscatter signal will return to line 66 during one of thepulses, causing circuit 114 to turn “ON”, thereby establishing normalanalyzer operation.

Still another input to OR gate 116 is a manual override pushbutton 124.When activated, pushbutton 124 forces laser 16 to the high outputintensity. This capability is provided for troubleshooting purposes.

Turning now to FIG. 8, the asymmetric transient discrimination featureof comparator/asymmetric discriminator circuit 114 will be described.The amplifier 64 is shown in more detail as a first stage transimpedanceamplifier and a second stage amplifier with gain. The circuit 114 isshown as three circuits 114 a, 114 b, 114 c connected in series. Circuit114 a, configured as shown, performs the comparator function describedpreviously. Adjustment of potentiometer 126 adjusts the electricpotential at the noninverting input of operational amplifier 128, whichelectric potential functions as the threshold referred to previously,against which the amplified detector output on line 66 is compared.Since operational amplifier 128 is wired as a comparator, it hasessentially a digital output. This digital output changes state veryrapidly every time the signal on line 66 crosses the threshold potentialat the noninverting input of op amp 128. Ignoring circuit 114 b for themoment, if circuit 114 a was directly connected to circuit 114 c, thecircuit 114 output on line 130 would respond equally as rapidly topositive-going and negative-going changes in the line 66 signal as itcrossed the threshold. However, as is apparent from the wiring of FETtransistor 132 in circuit 114 c, the polarity of the signal on line 130is opposite that of the signal at the output of op amp 128.

Circuit 114 b, however, discriminates between positive-going andnegative-going transitions. During normal analyzer operation, with aRayleigh scattering component above the threshold level, the output ofop amp 128 is LO, the potential at node 134 is LO, capacitor C1 is notcharged, and transistor 132 is off. If the Rayleigh scattering componentsuddenly drops below the threshold level, the output of op amp 128immediately goes HI. Diode D1 is reverse biased (nonconducting), and thecombination of resistors R1, R2, and capacitor C1 delay the turn-on oftransistor 132. The delay (“τ”) is proportional to (R1+R2)*C1. If theRayleigh scattering component stays below the threshold level for atleast the delay time τ, transistor 132 will turn on, causing the laser16 to shut down. If however the detected Rayleigh component returns to alevel above the threshold level before time τ has elapsed, op amp 128output will immediately go LO, diode D1 will be forward biased(conducting), and capacitor C1 will discharge rapidly through onlyresistor R2. Preferably, the value of R1 is much greater than R2. In apreferred embodiment R1=200 kΩ, R2=10 kΩ, and C1=10 picofarads.Preferred delay times τ are in the range of about 0 to 44 ms, and arepreferably programmable by computer 14 (e.g. by a computer-controlledswitch and one or more resistors in circuit 114 b that changes theeffective resistance in parallel with diode D1).

In this manner, circuit 114 b discriminates between a transitory loss inthe detected Rayleigh scattering component and a transitory appearanceof such component.

FIG. 9 depicts the output of pulse generator 122, the amplified detectoroutput on line 66, and the output of circuit 114 as waveforms 136, 138,140 respectively. Broken line 142 represents the threshold level set incircuit 114. At time t=0, the analyzer is powered up, the laser is off,and the probe is withdrawn from the specimen. At times t₁ and t₂, thepulse generator pulses the laser on, but only a very low Rayleighscattering component is detected since the probe is not contacting thespecimen. Waveform 140 therefore remains off. Between times t₂ and t₃,the probe is inserted into the specimen, so that at the next pulse ofwaveform 136 at time t₃, a Rayleigh scattering component above thethreshold is produced in waveform 138, and circuit 114 (waveform 140)rapidly responds. Between time t₄ and t₅, small bubbles passing throughdetection zone 54 cause transitory dips in waveform 142 below threshold142. The duration of such dips is less than τ, so the waveform 140remains unchanged. Between time t₅ and t₆, a larger bubble passesthrough detection zone 54, causing a dip in signal 138 with a durationlonger than τ, whereupon waveform 140 drops to zero. By the time t₆, thelarge bubble has passed zone 54 and the pulse of waveform 136 bringsback the Rayleigh scattering component in waveform 138 and the output ofcircuit 114. A transitory increase in waveform 138 between t₆ and t₇ hasno effect on waveform 140, since waveform 138 stays above threshold 142during that time. Between time t₇ and t₈, an interruption such as afiber break, fiber disconnection, or probe withdrawal occurs. Waveform138 responds immediately to the interruption, while waveform 140responds after the delay time τ.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. For example, analog circuits disclosed inthe preferred embodiment can be replaced with equivalent digitalcircuits, e.g., DSP filters.

What is claimed is:
 1. A method of fabricating a fiber optic probe,comprising: providing a probe terminus having a bore and a metalcoating; providing a plurality of metal-coated fibers having proximaland distal ends; positioning the distal ends of the plurality of fibersinside the bore; placing a braze material on the probe terminusproximate the bore; and heating the probe terminus, the fiber distalends, and the braze material in a vacuum oven to a temperaturesufficient to melt the braze material, thereby causing the brazematerial to wick between the fibers inside the bore.
 2. The method ofclaim 1, further comprising: connecting a probe body to the probeterminus; holding the proximal end of one of the plurality of fibers ina first connector, holding the proximal end of a second of the pluralityof fibers in a second connector, and holding the proximal end of theremaining plurality of fibers together in a third connector; and fixingthe first, second, and third connectors to the probe body.
 3. The methodof claim 1, further comprising: prior to the heating step, applying ananti-wicking agent to the plurality of fibers in a zone proximate thedistal ends.
 4. The method of claim 1, wherein the metal coating on theprobe terminus is limited to a vicinity in and around the bore.
 5. Themethod of claim 1, wherein the metal coating on the probe terminuscomprises gold and the plurality of metal-coated fibers have a goldcoating.
 6. The method of claim 5, wherein the braze material is abinary metal alloy.
 7. The method of claim 6, wherein the binary metalalloy includes silver and copper.
 8. A probe for in situ spectroscopicanalysis, comprising: a probe body having a distal end and a proximalend; a first optical fiber extending from the proximal end to the distalend; and a temperature sensor disposed in the probe body proximate thedistal end.
 9. The probe of claim 8, further comprising: a first fiberoptic connector disposed at the proximal end and holding the firstoptical fiber.
 10. The probe of claim 9, further comprising: a secondconnector coupled to the temperature sensor and carried by the probebody at the proximal end.
 11. The probe of claim 8, further comprising:a second optical fiber extending from the proximal end to the distalend; and a second fiber optic connector disposed at the proximal end andholding the second optical fiber.
 12. The probe of claim 11, furthercomprising: a third optical fiber extending from the proximal to thedistal end; and a third fiber optic connector disposed at the proximalend and holding the third optical fiber.
 13. The probe of claim 10,wherein the temperature sensor includes a thermocouple.